1. Field of the Invention
This invention relates to processes for the production of water soluble oxygenated hydrocarbons, e.g., ethanol, butanol, acetone, etc. by batch or continuous fermentation.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of industrial chemicals and liquid fuels from vegetative sources becomes increasingly attractive. Thus, for example, in some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain pertroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such renewable sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentaion of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy requirements for manufacturing ethanol and other industrial chemicals from biomass and consequently, little effort has been made to minimize the thermal expenditure for carrying out any of the discrete operations involved in the manufacture of these important materials from vegetative sources.
In a typical ethanol fermentation process, an aqueous solution of fermentable substrate, i.e., dextrose, and a quantity of fermenting microorganisms, i.e., yeast cells, are introduced into one or the first of a series of fermentation vessels wherein the sugar is metabolically converted by the yeast into product ethanol and carbon dioxide gas. Since the metabolic evolution of ethanol is exothermic, provision is made for the cooling of the fermentation medium to maintain a range of temperature conducive to high levels of ethanol production, i.e., from about 68.degree. F. to about 104.degree. F. and preferably from about 68.degree. F. to about 99.degree. F. The resulting dilute solution of ethanol (so-called "beer") which can contain up to about 12 weight percent ethanol is thereafter subjected to distillation if the ethanol is to be recovered in a more concentrated form. Additional ethanol can be recovered from the gases, largely carbon dioxide containing relatively minor amounts of ethanol, which are evolved during fermentation by scrubbing the gases with water. The carbon dioxide once freed of ethanol is then discharged to the atmosphere. The foregoing description with appropriate changes in fermentable substrate and fermenting microorganism is generally applicable to the production of other water soluble oxygenated hydrocarbons such as butanol and acetone, the by-product carbon dioxide also being vented to the atmosphere. In some known fermentation processes, the by-product carbon dioxide is returned to the fermentation vessel in order to maintain agitation which will prevent the fermenting microorganisms and other insolubles from settling.
In the process described in U.S. Pat. No. 3,117,005, carbon dioxide from one end of an enclosed fermentation vessel (for the production of potable beer) is passed through a heater and humidifier and returned in the heated condition to the top of the vessel where it ruptures the cells of the foam, made up mainly of protein, which accumulates above the surface of the fermentation medium. The disrupted foam then sinks to become part of the homogeneous mass at the bottom of the fermentation vessel which is said to absorb objectional flavors from the product beer.